1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transceivers (generally mobile) capable of being interrogated in a contactless and wireless manner by a unit (generally fixed), called a read/write terminal. The present invention more to specifically relates to transponders having no independent power supply. Such transponders extract the power supply required by the electronic circuits included therein from the high frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read only transponders, that is, adapted to operating with a terminal only reading the transponder data, or read/write transponders, which contain data that can be modified by the terminal.
2. Discussion of the Related Art
Electromagnetic transponders are based on the use of oscillating circuits including a winding forming an antenna, on the transponder side and on the read/write unit side. These circuits are intended to be coupled by a close magnetic field when the transponder enters the field of the read/write unit. The range of a transponder system, that is, the maximum distance from the terminal at which a transponder is activated (awake) depends, especially, on the size of the transponder antenna, on the excitation frequency of the coil of the oscillating circuit generating the magnetic field, on the intensity of this excitation, and on the transponder power consumption.
FIG. 1 very schematically shows, in a functional way, a conventional example of a data exchange system between a read/write unit 1 (STA) and a transponder 10 (CAR).
Generally, unit 1 is essentially formed of an oscillating circuit formed of an inductance L1 in series with a capacitor C1 and a resistor R1, between an output terminal 2p of an amplifier or antenna coupler 3 (DRIV) and a terminal 2m at a reference potential (generally, the ground). Amplifier 3 receives a high-frequency transmission signal Tx, provided by a modulator 4 (MOD). The modulator receives a reference frequency, for example, from a quartz oscillator 5 and, if necessary, a data signal to be transmitted. In the absence of a data transmission from terminal 1 to transponder 10, signal Tx is used only as a power source to activate the transponder if said transponder enters the field. The data to be transmitted come from an electronic system, generally digital, for example, a microprocessor 6 (xcexcP).
The connection node of capacitor C1 and inductance L1 forms, in the example shown in FIG. 1, a terminal for sampling a data signal Rx , received from a transponder 10 and intended for a demodulator 7 (DEM). An output of the demodulator communicates (if necessary via a decoder (DEC) 8) the data received from transponder 10 to microprocessor 6 of read/write terminal 1. Demodulator 7 receives, generally from oscillator 5, a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C1 and resistor R1 and not across inductance L1. Microprocessor 6 communicates (bus EXT) with different input/output (keyboard, screen, means of transmission to a provider, etc.) and/or processing circuits. The circuits of the is read/write terminal draw the energy necessary for their operation from a supply circuit 9 (ALIM), connected, for example, to the electric supply system.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a reception resonant circuit) intended for capturing the field generated by series oscillating circuit L1C1 of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned on the frequency of the oscillating circuit (L1, C1) of terminal 1.
Terminals 11, 12, of resonant circuit L2C2, which correspond to the terminals of capacitor C2, are connected to two A.C. input terminals of a rectifying bridge 13 formed, for example, of four diodes D1, D2, D3, D4. In the representation of FIG. 1, the anode of diode D1 and the cathode of diode D3 are connected to terminal 11. The anode of diode D2 and the cathode of diode D4 are connected to terminal 12. The cathodes of diodes D1 and D2 form a positive rectified output terminal 14. The anodes of diodes D3 and D4 form a reference terminal 15 of the rectified voltage. A capacitor Ca is connected to rectified output terminals 14, 15 of bridge 13 to store power and smooth the rectified voltage provided by the bridge. It should be noted that the diode bridge may be replaced with a single-halfwave rectifying assembly.
When transponder 10 is in the field of terminal 1, a high frequency voltage is generated across resonant circuit L2C2. This voltage, rectified by bridge 13 and smoothed by capacitor Ca, provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally include, essentially, a microprocessor (xcexcP) 17 (associated with a memory not shown), a demodulator 18 (DEM) of the signals that may be received from terminal 1, and a modulator 19 (MOD) for transmitting information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) extracted, by a block 20, from the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of transponder 10 are integrated in a same chip.
To transmit the data from transponder 10 to unit 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2C2. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals 14 and 15. Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (for example, 13.56 MHz). When switch T is closed, the oscillating circuit of the transponder is submitted to an additional damping as compared to the load formed of circuits 16, 17, 18, 19 and 20, so that the transponder draws a greater amount of power from the high frequency field. On the side of terminal 1, amplifier 3 maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and phase variation of the current through antenna L1. This variation is detected by demodulator 7 of terminal 1, which is either a phase demodulator or an amplitude demodulator. For example, in the case of a phase demodulation, the demodulator detects, in the half-periods of the sub-carrier where switch T of the transponder is closed, a slight phase shift (a few degrees, or even less than one degree) of the carrier of signal Rx with respect to the reference signal. The output of demodulator 7 (generally the output of a band-pass filter centered on the sub-carrier frequency) then provides an image signal of the control signal of switch T that can be decoded (by decoder 8 or directly by microprocessor 6) to restore the binary data.
It should be noted that the terminal does not transmit data while it receives some from a transponder, the data transmission occurring alternately in one direction, then in the other (half-duplex).
FIG. 2 illustrates a conventional example of a data transmission from terminal 1 to a transponder 10. This drawing shows an example of shape of the excitation signal of antenna L1 for a transmission of a code 0101. The modulation currently used is an amplitude modulation with a rate of 106 kbits/s (one bit is transmitted in approximately 9.5 xcexcs) much smaller than the frequency (for example, 13.56 MHz) of the carrier coming from oscillator 5 (period of approximately 74 ns). The amplitude modulation is performed either in all or nothing or with a modulation ratio (defined as being the difference of the peak amplitudes (a, b) between the two states (0 and 1), divided by the sum of these amplitudes) smaller than one due to the need for supply of transponder 10. In the example of FIG. 2, the carrier at 13.56 MHz is modulated, with a 106-kbit/s rate, in amplitude with a modulation rate tm of, for example, 10%.
FIG. 3 illustrates a conventional example of a data transmission from transponder 10 to terminal 1. This drawing illustrates an example of the shape of control signal VT of transistor T, provided by modulator 19, and of the corresponding signal Rx received by terminal 1. On the transponder side, the back modulation is generally of resistive type with a carrier (called a sub-carrier) of, for example, 847.5 kHz (period of approximately 1.18 xcexcs). The back modulation is, for example, based on a BPSK-type (binary phase-shift keying) coding at a rate on the order of 106 kbits/s, much smaller than the sub-carrier frequency. In FIG. 3, signal Rx has been shown as xe2x80x9csmoothedxe2x80x9d, that is, without showing the high frequency carrier (for example, at 13.56 MHz) ripple. In the example of FIG. 3, it has been assumed that each of the three shown bits is different from the preceding bit. Thus, for example, a code 010 is transmitted.
It should be noted that, whatever the type of modulation or back modulation used (for example, amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), this modulation or back modulation is performed digitally, by jumping between two binary levels.
The oscillating circuits of the terminal and of the transponder are generally tuned on the carrier frequency, that is, their resonance frequency is set on the 13.56-MHz frequency. This tuning aims at maximizing the power diffusion to the transponder, generally, a card of credit card size integrating the different transponder components.
As illustrated in FIG. 3, signal VT is formed of a pulse train at the sub-carrier frequency (for example, 847.5 MHz), a phase shift occurring upon each state change from one bit to the next bit. As concerns the signal recovered on the reader side, it appears not to have a xe2x80x9cdigitalxe2x80x9d form, which can make its decoding difficult. Indeed, the shape of signal Rx has, for each bit transmission time (9.4 xcexcs), a non-linear increase beginning (in capacitance charge) to a maximum approximately at two thirds of the duration of a bit, then an also nonlinear decrease. The enable time, that is, the time taken by signal Rx to reach a level decodable by the demodulator, is linked to the oscillating circuits being tuned. The need for power transfer for the remote supply, associated with the desired system range, requires a high quality factor, and thus that the oscillating circuits be tuned. Now, a high quality factor results in a small pass-band. This results in a limited data flow for the system. Generally, the quality factors are on the order of 10 for the reader and for the transponder.
The transponder may be formed by various objects (key ring, keys, etc.), but is most often, nowadays, in the form of a credit card integrating all the circuits and the antenna or inductance L2. For an information exchange with a reader or a terminal, the card is brought closer to antenna L1 of the reader. The distance between the reader and the card varies and, in some applications, a very close or tight coupling transmission is used, the antennas being distant from each other by less than approximately two centimeters. Such a tight coupling transmission may be used, for example, to enable a payment by means of a transponder, and thus guarantee that only the transponder that is closest to the terminal is recognized by said terminal.
A problem that is raised when the oscillating circuits are very close to each other is that, if they are substantially tuned, the power transmitted from the terminal to the transponder is such that the transponder heats up (antenna L2 is generally formed of one or several planar spirals at the card periphery). This thermal effect results in deforming the plastic card.
The present invention aims at providing a novel solution that overcomes the disadvantages of conventional solutions when a transponder is in very tight coupling relation with a read/write terminal.
The present invention aims, in particular, at reducing or minimizing the thermal effect linked to the remote supply of the transponder by the read/write terminal.
The present invention also aims at enabling an increase of the data transmission rate when the transponder is very close to the terminal.
The present invention also aims at providing a solution that requires no structural modification of the reader or terminal.
A feature of the present invention is to detune the oscillating circuit of the transponder when it is in very close or tight coupling relation with a read or read/write terminal.
A frequency detuning of an electromagnetic transponder is known from document WO-A-98/29760. This document provides for the antenna of a transponder to be xe2x80x9cdetuned in frequency or mismatched in impedance, so that the transponder and its electronic circuit absorb less radio field and power. Thus, another transponder located in the vicinity of the mismatched or detuned transponder can receive enough of the radio field to operate properly. The transmission system can then detect or consult this other transponder as if it were alone in the fieldxe2x80x9d of the transmitter. Still according to this document, the mismatch means are used xe2x80x9cwhen the transponder is in an unselected state to limit the power and/or field absorption by the transponder in the unselected statexe2x80x9d.
The solution advocated by this document amounts to detuning the transponders that are relatively remote from the terminal to maximize the power received by the closest transponder meant to communicate with the terminal. Such a solution does not solve the above-mentioned tight coupling problems. Indeed, the transponder that remains tuned is the selected transponder.
Conversely to this document, the present invention provides a detuned operation in tight coupling. Thus, a feature of the present invention is to provide, for a tight coupling information transmission, a detuned operation of the oscillating circuit of an electromagnetic transponder remotely supplied by a terminal.
The present invention takes account of the fact that the remote supply power recovered on the transponder side is not a monotonic function of the distance that separates the transponder from the terminal.
Indeed, when the oscillating circuits are tuned on the remote supply carrier frequency, if the transponder comes close to a terminal, the remote supply amplitude starts increasing from the system range limit (on the order of some ten centimeters). This amplitude transits through a maximum (critical coupling position) then starts decreasing again when the transponder becomes very close (approximately less than 2 centimeters). For this reason, in particular, it is not provided in conventional systems to make the power of the terminal dependent from the distance at which the transponder is.
The critical coupling position corresponds to the distance at which the coupling between the transponder and the terminal is optimized by a maximum remote supply amplitude received by the transponder when the oscillating circuits of the terminal and of the transponder are both tuned on the remote supply carrier frequency. In other words, the critical coupling frequency corresponds to the distance at which the remote supply power is maximum for a minimum coupling factor, the coupling factor being the ratio of the mutual inductance on the square root of the product of the inductances of the oscillating circuits.
When the oscillating circuit of the transponder is detuned from the remote supply carrier frequency, the power received by the transponder increases as the distance from the terminal decreases, but with a reduced range. In this case, there also is a distance at which the received power is maximum for a given detuning condition. This is an optimal coupling, the critical coupling position being the optimal coupling condition when the two oscillating circuits are tuned on the carrier frequency. It should be noted that the optimal coupling coefficient between the two oscillating circuits depends not only on inductances L1 and L2, on capacitors C1 and C2, and on the frequency (which here is a fixed frequency and corresponds to the carrier frequency), but also on series resistance R1 of the terminal, and on the load of the oscillating circuit of the transponder, that is, on the equivalent resistance of the circuits (microprocessor, etc.) and on the back modulation means (for example, resistor R, FIG. 1), added in parallel on capacitor C2 and on inductance L2. This equivalent resistor will be designated hereafter as R2.
Thus, the contactless and wireless transmission system operates even if one of the oscillating circuits is detuned, provided that the antennas are very close to each other.
More specifically, the present invention provides an electromagnetic transponder of the type including an oscillating circuit upstream of a rectifying means adapted to providing a D.C. supply voltage to an electronic circuit, the electronic circuit including means for transmitting digitally-coded information, and the transponder including means for detuning the oscillating circuit with respect to a determined frequency, the means for detuning the oscillating circuit being used when the transponder has to transmit information while it is very close to a read/write terminal.
According to an embodiment of the present invention, the determined frequency corresponds to the frequency of a carrier for remotely supplying the transponder, coming from the terminal.
According to an embodiment of the present invention, the means for detuning the oscillating circuit are formed of a switched capacitor, in parallel with an inductive element of the circuit, the rectifying means being formed of a one-way conduction element.
According to an embodiment of the present invention, the means for detuning the oscillating circuit are formed of two capacitors respectively associated with each end terminal of an inductive element of the second oscillating circuit, each capacitor being connected in series with a switching means, a reference terminal of which is connected to a reference supply potential of the electronic circuit, downstream of the rectifying means.
According to an embodiment of the present invention, the means for detuning the oscillating circuit are controllable between two positions to enable a selection between an operation of the transponder tuned on the determined frequency, or detuned from this frequency.
According to an embodiment of the present invention, the means for detuning the transponder are also used to detect the distance separating it from a read/write terminal.
According to an embodiment of the present invention, the transponder further includes two resistive modulation means, in parallel on a capacitor for smoothing the rectified voltage provided by the rectifying means, each modulation means being dedicated to one of the tuned or detuned operating modes of the transponder.
The present invention also relates to a system of wireless and contactless data transmission between a terminal of generation of an electromagnetic field and at least one transponder having no independent supply means.
According to an embodiment of the present invention, the data transmission rate from the transponder to the terminal is different according to whether the oscillating circuit of the transponder is or is not tuned to the determined frequency.
According to an embodiment of the present invention, the data transmission rate from the terminal to the transponder is different according to the oscillating circuit of the transponder is or is not tuned to the determined frequency.
The foregoing objects, features and advantages of the present invention, will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.